We are continuing our studies on the contractile proteins from Limulus muscle and comparing our results with those obtained on comparable proteins from Mercenaria, lobster and rabbit. Our immediate goals are 1) purification of Limulus myosin, 2) preliminary characterization of its ATPase activity particularly with respect to its activity with Ca and/or Mg, and 3) the effect of Limulus and/or rabbit actin on the myosin ATPase. We also plan to begin studies on the subunits of Limulus myosin. Work will continue on the purification of tropomyosin and actin from this animal as well as from lobster.